In the midst of close-up of a global environmental problem such as increase in carbon dioxide, photovoltaic power generation has been drawn attention again, as well as effective utilization of hydraulic power, wind power, geothermal heat or the like.
The photovoltaic power generation is the one using a packaged solar cell module, obtained in general by protecting a solar cell element of silicon, gallium-arsenic, copper-indium-selenium or the like with an upper transparent protecting material and a lower substrate protecting material, and fixing the solar cell element and the protecting material with an encapsulant made of a resin, and since it can be arranged in a decentralized state at a place where electric power is necessary, although scale thereof is smaller as compared with hydraulic power, wind-power or the like, research and development has been promoted aiming at performance enhancement of power generation efficiency or the like, and price reduction. In addition, by adoption of a policy to subsidize the installment cost, as an introduction promotion business of a residential house photovoltaic power generation system by state or local governments, prevalence thereof has gradually been progressing. However, still more cost reduction is required for further prevalence thereof, and effort has, accordingly, been continued in a step-by-step fashion not only to develop a solar cell element using a novel material instead of conventional-type silicon or gallium-arsenic, but also to further reduce production cost of the solar cell module.
As requisite of the solar cell encapsulant composing the solar cell module, good transparency has been required, so as not to decrease power generation efficiency of the solar cell, and to secure incidence amount of sunlight. In addition, the solar cell module is usually installed outdoors, which raises temperature caused by exposure to sunlight for a long period of time. To avoid such a trouble as deformation of the module by flowing of the encapsulant made of a resin, caused by increase in temperature, it should be the one having heat resistance. In addition, thinning trend has been progressing year by year, to reduce material cost of the solar cell element.
At present, as the encapsulant of the solar cell element in the solar cell module, in view of flexibility, transparency or the like, an ethylene vinyl acetate copolymer, having high content of vinyl acetate, has been adopted as a resin component, where an organic peroxide is used in combination, as a cross-linking agent (for example, see PATENT LITERATURE 1).
And, in an encapsulation work of the solar cell element, the solar cell element is covered with an encapsulant made of a resin, then it is heated for several minutes to ten and several minutes for tentative adhesion, and is heat-treated at high temperature, where the organic peroxide decomposes in an oven, for several minutes to 1 hour for adhesion (for example, see PATENT LITERATURE 2).
However, to suppress production cost of the solar cell module, it has been required to further shorten time required in the encapsulation work, and a solar cell encapsulant has been proposed, which is composed of an α-olefin copolymer which is amorphous or low crystalline with a crystallinity of 40% or less, instead of the ethylene vinyl acetate copolymer, as a resin component of the encapsulant (see PATENT LITERATURE 3). In this PATENT LITERATURE 3, there has been exemplified production of a sheet at a processing temperature of 100° C., using a profile extruder, by mixing an organic peroxide to an amorphous or low crystalline ethylene butene copolymer, however, because of low processing temperature, sufficient productivity has not been obtained.
There was also the case in the solar cell module, where the module may deform, by temperature increase caused by exposure to sunlight for a long period of time, due to installment outdoors as above-described, followed by resultant decrease in adhesion strength between a glass substrate and the encapsulant made of a resin, separation of the encapsulant made of a resin from the glass substrate, and intrusion of air or moisture into space there between. The PATENT LITERATURE 1 has described to blend a silane coupling agent to an encapsulating resin, however, it relates to the solar cell module using a flexible substrate such as a fluorocarbon resin film, and has not clarified detail of the silane coupling agent. The above PATENT LITERATURE 2 has also described to blend a silane coupling agent to the encapsulating resin, however, it relates to the solar cell module using an EVA film and an FRP substrate, and adhesion property with the substrate is not sufficient.
In addition, as the encapsulant of the solar cell module, there has been proposed a polymer material containing an polyolefin copolymer which satisfies one or more requisites among (a) a density of less than about 0.90 g/cc, (b) a 2% secant modulus of less than about 150 megapascal (mPa), measured in accordance with ASTM D-882-02, (c) a melting point of lower than about 95° C., (d) an α-olefin content of at least about 15% by weight and lower than about 50% by weight, based on weight of a polymer, (e) a Tg of lower than about −35° C. and (f) an SCBDI of at least about 50 (see PATENT LITERATURE 3).
In the solar cell module, there is tendency for also the solar cell encapsulant to become thinner accompanying with thinning of the solar cell element. In this case, it has been a problem of easy break of a wiring caused when impact is applied thereto from the upper protecting material side or the lower protecting material side of the solar cell encapsulant. Use of the polymer material of the PATENT LITERATURE 4 is capable of increasing rigidity of the encapsulant, however, it deteriorates cross-linking efficiency, and was thus not practical.
As just described, conventional technology has not provide the resin composition for a solar cell encapsulant with superior productivity, heat resistance, transparency, flexibility and adhesive property to the glass substrate, along with good balance between rigidity and cross-linking efficiency.